1. Technical Field of the Invention
The present invention relates in general to interferometers, and in particular, to interferometer architectures, fabrications and applications.
2. Description of Related Art
Interferometers split a beam of light into two paths (or arms), bouncing them back and recombining them to detect an interference pattern produced as a result of the recombination. The different arms may be of different lengths or composed of different material to create alternating interference fringes on a back detector.
One of the most significant interferometer-based imaging techniques developed in the past several decades is Optical Coherence Tomography (OCT). The appearance of OCT in the 1990's filled the gap in biomedical imaging between high resolution small penetration depth optical confocal microscopy and low resolution high penetration depth HF ultrasound. OCT was originally developed in communication systems for the characterization of integrated optical or fiber optical components. After OCT was proposed by MIT for tomography of the human eye, the number of companies developing medical instruments using OCT has increased exponentially.
There are several different OCT techniques, but all of the techniques use a Michelson interferometer in which the sample under test is inserted into one of the two arms of the interferometer. The second arm, called the reference arm, typically includes a conventional mirror (e.g., a metallic mirror). The output of the interferometer delivers the required information signal.
One common OCT technique is the Time Domain OCT or simply TD OCT. In the TD OCT, the signal reflected from the reference arm has the same time delay as the signal coming from the sample, so that there are two mutually coherent signals. In addition, the input source is a low coherence length source (a source with a wide spectrum), and thus the output is maximal only within the coherence length of the source. The amplitude of this maximum represents the amplitude of reflection from a point in the sample corresponding to the position of the reference arm mirror. If this position is scanned, the output power with each reference arm mirror position is measured and then the resulting output powers are used to construct an interferogram. By this way, a depth tracking signal is obtained in the TD OCT technique. However, the coherence length of the input source may only be few microns. As such, the resolution limit of the TD OCT technique may be limited.
In a different technique, called the spectral domain technique (SD OCT), the reference arm has a fixed mirror and the reflected signal from the interferometer is passed through an optical spectrometer and then inverse Fourier Transformed (IFFT). The signal obtained after the IFFT represents the depth tracking signal in the spectral domain technique. The corresponding maximum depth is determined by the resolution of the spectrometer used as:
      z    max    =            λ      2              4      ⁢      n      ⁢                          ⁢      δ      ⁢                          ⁢      λ      where λ is the wavelength, δλ is the spectrometer resolution and n is the average sample refractive index.
In a similar technique, called the frequency domain OCT (FD OCT), the low coherence source is replaced by a tunable laser source, the output of the interferometer is compared against the different optical frequencies and an IFFT is used to gain the depth tracking signal. In the FD OCT, the resolution is represented by the maximum tuning range of the laser used.
Traditional OCT devices are constructed using conventional metallic mirrors. However, with the introduction of Micro Electro-Mechanical Systems (MEMS) technology, there has been an increasing interest in developing a MEMS OCT device. MEMS refers to the integration of mechanical elements, sensors, actuators and electronics on a common silicon substrate through microfabrication technology. For example, the microelectronics are typically fabricated using an integrated circuit (IC) process, while the micromechanical components are fabricated using compatible micromachining processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical components.
The fabrication of an OCT device using MEMS technology could be a great advantage as it enables a low cost, low weight and size OCT head to be produced, which is desired in biomedical applications. The OCT head is usually portable, and thus lends itself to MEMS technology. However, there are several technical challenges to introducing an OCT head enabled by MEMS technology.
One of the main challenges is the scanning depth. In TD OCT, the scanning depth is determined by the travel range of the mirror. However, the state of the art travel range of MEMS mirrors is limited (<0.5 mm), which greatly limits the applicability of MEMS technology in TD OCT. In addition, although the reference mirror is fixed in SD OCT, the scanning depth is limited by the resolution of the spectrometer. This presents a difficult challenge since MEMS spectrometers typically have limited resolutions as a result of the small mirror travel range in Fourier Transform Spectrometers or the number of pixels in dispersive spectrometers.